1. Field of the Invention
The present invention relates to a light emitting module.
2. Related Background Art
In 1.55 xcexcm-band WDM systems, the wavelength spacing between adjacent channels is stipulated as 0.8 nm. This requires that the absolute accuracy of each channel wavelength should be controlled within the precision of xc2x10.1 nm or higher. DFB semiconductor lasers and DBR semiconductor lasers may be utilized for 1.55 xcexcm-band WDM systems.
These semiconductor lasers provide a sharp oscillation spectrum, but their oscillation wavelength is determined by a diffraction grating fabricated in a laser chip at the manufacturing stage of semiconductor laser. It was not easy to yield a desired oscillation wavelength stably and accurately, because characteristics of the diffraction grating were affected by manufacturing process factors.
For implementation of it, there is the following attempt. A semiconductor laser chip is assembled to obtain a light emitting module. During operation of the module, output light from the light module is branched and this branch light is monitored by a large-scale apparatus like an optical spectrum analyzer. According to the monitor information, temperature or injection current of the semiconductor laser chip is adjusted.
However, in the wavelength division multiplexing (WDM) systems, it is not easy to realize a light emitting module applicable to the WDM systems because a plurality of wavelengths are used to transmit data in 16 channels or 32 channels.
It is, therefore, an object of the present invention to provide a light emitting module that permits easy adjustment of the wavelength of light generated in the light emitting module, without using any large-scale system like the optical spectrum analyzer.
In order to realize the light emitting module capable of attaining this object, the inventors conducted a variety of studies, e.g., on the light emitting modules incorporating the semiconductor laser. In order to adjust the oscillation wavelength of the light emitting module while operating the light emitting module, it is necessary to monitor the wavelength. For extracting oscillating light, an optical branching device, such as an optical coupler, needs to be coupled to output of the light emitting module. However, if the device having this function is used, the scale of the WDM systems will be large.
According to these studies, it became apparent that there were technical problems as follows. (1) There is a need for use of optical coupling means for obtaining monitor light to monitor light from the semiconductor light emitting element such as the semiconductor laser. (2) There is a need for use of separating means for separating the light from the coupling means into wavelength components. (3) There is a need for use of converting means for converting the light components from the separating means into electric signals.
In view of these problems, the inventors accomplished the present invention in structure as follows.
A light emitting module of the present invention comprises a semiconductor light emitting device, a photodetection device, an etalon device, and collimating means. The semiconductor light emitting device has first and second end faces. The photodetection device has first and second photodetectors optically coupled to the first end face of the semiconductor light emitting device. The etalon device has a first portion having a first thickness and a second portion having a second thickness. The first portion of the first thickness is provided so as to be located between the first end face of the semiconductor light emitting device and the first photodetector. The second portion of the second thickness is provided so as to be located between the first end face of the semiconductor light emitting device and the second photodetector. The first thickness of the etalon device is different from the second thickness of the etalon device. The collimating means functions to provide substantially collimated light for the etalon device that receives the light from the semiconductor light emitting device.
In the etalon device, the thickness of the portion located between the first end face of the semiconductor light emitting device and the first photodetector is different from that of the portion located between the first end face of the semiconductor light emitting device and the second photodetector. Light of different wavelength components passes through portions of the different thicknesses corresponding to the wavelength components in the etalon device. Therefore, if the wavelength components of light from the semiconductor light emitting device is changed, intensities of light passing through the particular portions of the etalon device varies in response to the change. This variation is converted into electric signals by the first photodetector and the second photodetector. Changes of these electric signals indicate the change of wavelengths in the light generated in the semiconductor light emitting device.
A difference signal between these electric signals represents a direction of the change of wavelengths in the light. By controlling the semiconductor light emitting device portion so as to keep this difference signal constant, it becomes feasible to keep the wavelength constant in the light generated in the semiconductor light emitting device.
The features according to the present invention as described below can be combined with the above-stated invention. The features according to the present invention as described below can be also combined with each other to enables the module to obtain actions and effects of the respective features and also obtain actions and effects achieved by the combination.
In the light emitting module, the etalon device has first and second surfaces. The first surface is arranged so as to be opposed to the second surface. The first and second surfaces are positioned so that an interval between them in the first portion is the first thickness. The etalon device has third and fourth surfaces. The third surface is provided so as to be opposed to the fourth surface. The third and fourth surfaces are located so that an interval between them in the second portion is the aforementioned second thickness. This configuration can provided the etalon device having the first and second thicknesses.
In the light emitting module of the present invention, the etalon device has a light receiving surface and a light outgoing surface. The light receiving surface is arranged so as to receive the light from the first end face of the semiconductor light emitting device, and the light outgoing surface is arranged so as to face the light receiving surface. The light receiving surface includes first and third faces. The light outgoing surface includes second and fourth faces. In the light emitting module of the present invention, the light receiving surface is inclined to the light outgoing surface. Because of this inclination, the distance between the light receiving surface and the light outgoing surface increases in a direction directed from the first portion to the second portion of the etalon device.
The etalon device has the light receiving surface and the light outgoing surface, the distance of which is changed in the first direction. When the etalon device is moved relative to the first and second photodetectors in the first direction, the transmission spectra achieved by the first and second portions of the etalon device is changed. This change results in changing the wavelength components of light received through the etalon device by the first and second photodetectors. The center wavelength of light generated in the semiconductor light emitting device can be adjusted by making use of this change. The transmitting peak wavelengths of the etalon device, which is utilized for adjusting the center wavelength of the light generated by the semiconductor light emitting device, can also be adjusted by rotating the etalon device. In the light emitting module, the etalon device is arranged as inclined relative to the semiconductor light emitting device in a direction perpendicular to a second direction, which is defined as a direction directed from the first portion to the second portion of the etalon device. This can reduce the amount of light reflected by the etalon device back to the semiconductor light emitting device.
In the light emitting module, each of the first and second photodetectors can be a photodiode element. In the light emitting module of the present invention, the first and second photodetectors can be attached to the etalon device. The light-emitting module of the present invention can further comprise an aperture device. The aperture device has one or more apertures located between each of the first and second photodetectors and the semiconductor light emitting device. The aperture device defines a position(s) on the etalon device at which the light should be transmitted. This determines wavelength regions of light received by the first and second photodetectors. The aperture device can reduce optical reflection from the first and second photodetectors to the semiconductor light emitting device.
In the light emitting module, the collimating means includes an optical lens. The collimating means can include an optical lens, such as convex lenses or concave lenses, but it is not limited to these examples. The collimating means includes an optical circuit. The optical circuit has an optical branching waveguide and an optical waveguide for guiding the light from the semiconductor light emitting device to predetermined positions on the etalon device.
The light emitting module further comprises means for reducing optical returning from at least either one of the first and second photodetectors and the etalon device through the optical lens to the semiconductor light emitting device. In the light emitting module of the present invention, the lens has a size determined so as to reduce the optical returning from at least either one of the first and second photodetectors and the etalon device to the semiconductor light emitting device. The size means at least either one of a height and a width of the lens. The lens has a cut face extending in a direction of the optical axis of the lens. When the optical lens has the cut surface, the height of this optical lens can be set low. In the light emitting module of the present invention, the lens has a shielding portion provided so as to reduce the optical returning from at least either one of the first and second photodetectors and the etalon device to the semiconductor light emitting device. This can decrease the amount of light which is incident on the semiconductor light emitting device through the optical lens and is reflected by the etalon device and the first and second photodetectors.
In the light emitting module, the etalon device preferably receives incident light in a range of an angle not more than 85xc2x0 and/or in a range of an angle not less than 95xc2x0 where the angle is formed with respect to an axis extending perpendicularly to a direction in which the first and second photodetectors is arrayed.
The light emitting module further comprises wavelength adjusting means for changing a wavelength of light generated by the semiconductor light emitting device in response to signals from the first and second photodetectors. This wavelength adjusting means can adjust the temperature of the semiconductor light emitting device according to the electric signals from the first and second photodetectors, and thereby change the wavelength of light generated by the semiconductor light emitting device. For examples, the wavelength adjusting means includes a thermoelectric cooler capable of adjusting the temperature of the semiconductor light emitting device and an optical waveguide having electrodes capable of changing the refractive index of a wave guide by an applied electric field.
In the light emitting module, the wavelength adjusting means can comprise a control circuit and temperature changing means. The control circuit can generate a control signal for adjusting, in response to the electric signals from the first and second photodetectors, the wavelength of light generated in the semiconductor light emitting device. The temperature changing means can adjust the temperature of the semiconductor light emitting device according to the control signal. The control circuit can be arranged inside or outside the light emitting module as required.
The following configuration can be applied to the light emitting module. The etalon device can provide light including a first wavelength component in a predetermined oscillation spectrum of light received from the semiconductor light emitting device, and provide light including a second wavelength component different from the first wavelength. The first and second photodetectors can provide first and second electric signals corresponding to the light of the first and second wavelengths, respectively. The temperature changing means can adjust the temperature of the semiconductor light emitting device in response to a difference signal generated from the first electric signal and the second electric signal. A driving circuit can adjust a driving current for controlling the optical output of the semiconductor light emitting device, in response to a sum signal generated from the first electric signal and the second electric signal. The center wavelength of light generated in the light emitting device is preferably located between the first wavelength and the second wavelength.
The light emitting module comprises the semiconductor light emitting device, the etalon device, and the first and second photodetectors and may further comprise a control circuit and a temperature controller. The etalon device acts as a wavelength filter whose transmission spectral characteristics differ according to transmission positions thereof. Accordingly, the intensities of light transmitted according to the transmission spectra at the respective transmitting received by the etalon, from the semiconductor light emitting device. The control circuit can generate a difference signal between electric signals from the first and second photodetectors and can also generate a sum signal thereof. The temperature controller changes the temperature of the semiconductor light emitting device, in response to the difference signal from the control circuit.
In the light emitting module, the predetermined thickness d of the etalon device should be determined as follows:
d=c/(2xc2x7nxc2x7kxc2x7xcex4xcexdWDM),
where
k=1xe2x88x92(dxcexd/dT)etalon/(dxcexd/dT)LD,
(dxcexd/dT)etalon: change rate of light frequency against temperature, wherein the light interferes at the position of the thickness d of the etalon device,
(dxcexd/dT)LD: change rate of light frequency against temperature, wherein the light is generated in the semiconductor light emitting device,
xcex4xcexdWDM: wavelength division multiplexing (WDM) frequency spacing.
When the oscillation wavelength of the semiconductor light emitting device is changed by changing the temperature of the semiconductor light emitting device, this etalon device allows the oscillation wavelength of the semiconductor light emitting device to change by the spacing between oscillation wavelengths of light which the semiconductor light emitting device is capable of generate.
The features according to the present invention as described below are applied to the etalon device in which the light receiving surface is inclined relative to the light outgoing surface so that the interval between the light receiving surface and the light outgoing surface increases in the direction from the first portion to the second portion of the etalon device.
In the light emitting module, the interval between the first photodetector and the second photodetector is determined as follows: an absolute value of a slope at a zero point of a difference spectrum is not less than 200 (%/nm). The difference spectrum is defined by a difference between a first transmission spectrum in the first portion of the etalon device and a second transmission spectrum in the second portion thereof.
In the light emitting module, the reflectivity of each of the light receiving surface and the light outgoing surface is in a range of not less than 30% nor more than 60%. The distance L (mm) between the first photodetector and the second photodetector satisfies the following relations where the reflectivity of the etalon device is R(%):
xe2x88x920.01xc3x97R+0.6xe2x89xa6Lxe2x89xa6xe2x88x920.01xc3x97R+0.8, and 0.2xe2x89xa6L.
In the region specified by these relations, the difference spectrum characteristics demonstrate excellent linearity. The wavelengths of the light transmitted by the etalon device are selected by changing the spacing between the photodetectors. This selection permits change in the profile of the difference spectrum. This change optically enhances detection sensitivity of wavelength shift in the semiconductor light emitting device.
In the light emitting module, each of the first photodetector and the second photodetector has a first width and a second width and is formed so that the first width is smaller than the second width. The first width is defined as a length in a direction in which the light receiving surface of the etalon device is inclined relative to the light outgoing surface thereof. The second width is defined as a length in a direction perpendicular to the foregoing direction, and. This configuration improves monochromaticity of received light.
In the light emitting module, the semiconductor light emitting device includes a semiconductor laser device having first and second end faces. This configuration provides a semiconductor laser module.